23 Soil Formation

Babita Khosla

Soil formation

Objectives:

To study the process of formation of Soil

To study the mechanism involved in Soil formation To study the different types of Soil weathering

24.1 Soil formation

Soils development results by the intermesh of 5 factors- climate, Parent material, relief organisms, and time (Fig-24.1).

1. Parent material

The material from which the soil has developed is called parent material and varies from solid crust to deposits of alluvium as well as boulder clay. It has been defined as ‘the initial state of the soil system’. The parent material influence the soil in a wide number of ways

• Structure
• Color
• Texture
• Mineral composition
• Permeability/drainage

Such soil was developed on Old Red Sandstone and so has copied its distinctive color from its parent material.

2. Climate

Climate is one of the most important soils forming factor (soils formed from the similar parent material under different climates contrast). Climate controls the type and rate of soil formation and it is also considered as the major determinant of distribution of vegetation. Soil climate comprises of two main components- Temperature and moisture (precipitation), which influences the process of evaporation. Leaching of the soil occurs, when precipitation exceeds evaporation. Temperature governs the rate of reactions; chemical as well as biological decomposition and thus has an influence on humification and weathering.

3. Organisms (Flora, fauna and soil organisms)

Organisms affecting the development of soil range from microscopic bacteria to animals including man. Microorganisms such as bacteria and fungi helps in the degradation of plant litter. The litter is finally mixed into the soil by microbes such as beetles and worms. Horizons of soil are less distinct when the activity of soil organism is high. Higher plants affect the soil in a number of ways. The makeup of the soil humus is estimated by the resultant litter inputs as well as vegetation cover. Roots supplement as dead roots to the soil, combine the soil particles collectively and reorganizes and compress the soil.

4. Relief (Topography and landforms)

Relief is not stagnant but a dynamic system and its study is known as geomorphology which influences the formation of soil in different ways-

• It regulates the thickness of soil profile i.e. when the angle of slope increases the erosion hazard also increases.
• It also influences the climate which is itself an important soil forming factor.
• Gradient influences the percolation, run-off and mass movement
• The aspect which creates microclimatic conditions is also varied.

5. Time

Development of soils is a very slow process. For example it was found that In Britain the development of 10 mm of soil takes about 400 years. Young soils maintain various features of the parent material. Over the passage of time the other features are also added, resulting from the addition of organic matter as well as microbial activity. The Britain soils are relatively young as they are largely post-glacial. An important characteristic of these soils is that they pass through various stages during their development, thus resulting in a deep differentiated profile.

24.2 Mechanism of Soil formation

Soil development occurs through a series of variations. All the soils evolve from their parent material, which is a deposit on earth’s surface. Parent material can be either weathered bedrock or small particles brought about by strong winds, moving glaciers or flowing rivers (24.2). The simple compounds discharged by the process of weathering serves as food sources for various soil organisms like bacteria and fungi. The remnants of these dead organisms incorporate in the parent material which in turn converts into organic matter also called as humus.

24.3 Weathering

Weathering is defined as the breakdown and alteration of minerals and rocks near the Earth’s surface into different products that are in equanimity with the conditions found in environment. It is the initial step for many biogeochemical and geomorphic processes. The products of weathering are the leading source of sediments for erosion as well as deposition. A variety of different sedimentary rocks consists of fragments which are weathered, eroded, transported, and finally deposited in basins. Weathering also results in the soil formation by providing mineral particles such as sand, silt, and clay. Weathering can be categorized in three broad categories: chemical, physical and biological (Fig-24.3).

24.3.1 Chemical Weathering

Chemical weathering includes the changes in chemical as well as mineralogical configuration of weathered material. Many different processes results in chemical weathering. The most common processes included in chemical weathering are oxidation, hydrolysis, reduction, carbonation,hydration, and solution. Hydrolysis is the weathering process that results when two surfaces of water and compound encounter. It includes the reaction between mineral ions and the water ions (OH- and H+), and finally the disintegration of the rock surface by formation of new compounds, and by enhancing the pH of the solution by the release of the hydroxide ions. Hydrolysis is markedly effective in the weathering of minerals like silicate and alumino-silicate since they have electrically charged crystal surfaces.

Oxidation reactions develop between compounds and oxygen. The final result of this reaction includes the removal of one or more electrons from the compound, which makes the structure less rigid and highly unstable. The most common oxides of iron and aluminum, and their respective red and yellow staining of soils is a common example in tropical regions with high temperature and precipitation.

Reduction is just the converse of oxidation, which is caused by addition of one or more electrons producing a more stable compound.

2Fe2O3 (Hematite) – O2  →       4FeO (Ferrous oxide reduced form)

Hydration includes the definite attachment of H+ and OH- ions to a reacted compound. In various situations the H+ and OH- ions forms the structural part of the crystal lattice of the mineral. Hydration also results in acceleration of various decomposition reactions by expanding the crystal lattice by increasing the surface area for reaction.

a) 2Fe2O3 + 3HOH→2Fe2O3 .3H2O (Hematite) (Red) (Limonite) (Yellow)

b) Al2O3 + 3HOH →Al2O3 .3H2O(Bauxite)(Hyd. aluminium Oxide)

1. c) CaSO4 + 2H2O → CaSO4 .2H2O

(Anhydrite)                          (Gypsum)

2. d) 3(MgO.FeO.SiO2) + 2H2O →3MgO.2SiO2.2H2O + SiO2 + 3H2O

(Olivine)(Serpentine)

Carbonation is the reaction of carbonate and bicarbonate ions with minerals. The formation of carbonates generally occurs as a result of other chemical processes. Carbonation process is most active when the reaction environment has sufficient carbon dioxide. The production of carbonic acid, which is a product of carbon dioxide and water, is essential in the solution of carbonates as well as the decomposition of mineral surfaces as it is acidic in nature.

The reactions as follows:

CO2 + H2O → H2CO3

Carbon dioxide + water → carbonic acid

H2CO3 + CaCO3 → Ca (HCO3)2

Carbonic acid + calcium carbonate → calcium bicarbonate Carbonation on the surface of well-jointed limestone results in a dissected limestone pavement. This process is highly effective along the joints, widening and deepening them.

24.3.2 Physical Weathering

Physical weathering involves the disintegration of mineral or rock material by many mechanical processes brought about by a different causes. Some of these forces develop within the rock or mineral, whereas others are applied externally. Both these stresses results into strain and finally rupturing of the rock. These processes include mechanical rupture such as abrasion, wetting as well as drying and ice formation. Freezing results into 9% volumetric change of water from liquid to solid. The reasonable volumetric change on freezing has potentially a great rupturing effect. Several researchers have found that in laboratory as well as in field, the frost action plays a dominant role in weathering in temperate and Polar Regions of the Earth. The threshold temperature for frost action is approximately – 5° C, as the most effective rupturing occurs at this temperature. Abrasion results when some force causes the two surfaces of rocks to come together resulting in mechanical wearing or grinding of their surfaces. Collision among rock surfaces normally results by the erosional transport of material by wind, water, or ice. One of the most intensely topics in rock weathering research also known as insulation weathering occurs by the physical breakdown of rock by their expansion and contraction due to diurnal temperature changes due to the physical inability of rocks to conduct heat well. Such inability to conduct heat results in different rates of expansion and contraction. Therefore the rock surface expands more than its interior, and this stress eventually causes the rupturing of rock. Differential expansion as well as contraction occurs due to variation in the colors of mineral grains in rock. Dark colored grains will expand much more than light colored grains because of their absorptive properties. Thus, in a rock diffused with various colored grains, rupturing can occur at different rates at the various mineral boundaries. Alternate wetting and drying of rocks, also known as slaking, is a very important factor in weathering. Slaking occurs by the mechanism of “ordered water”, which is the aggregation of consecutive layers of water molecules struck between the mineral grains of a rock. The increase in thickness of the water pulls the rock grains apart with higher tensional stress. Recent advancement has shown that slaking in collaboration with dissolved sodium sulfate can split the samples of rock in only twenty different cycles of wetting and drying.

24.3.3 Biological Weathering

Biological weathering results into disintegration of rock and mineral due to the chemical or physical agents of an organism. The types of organisms that can cause weathering range from bacteria to plants to animals. Biological weathering involves processes that can be either chemical or physical in character. Bio-chemical processes, root penetration, and animal burrowing are some of the processes determining biological weathering. Bio-chemical action plays an important role by emitting organic compounds. The organic compounds have acidifying molecules that corrode rock minerals and makes them weak and more prone to disintegration. Following are main types of biological weathering-

Growing Plant Roots: It is commonly found that some roots grow within the face of a rock. Such plant activity results in biological weathering. The roots of plants and trees pass into the soil for nutrients and water. As the roots move into the soil, they go through cracks or joints in the rocks and as they grow the crack also widens resulting into disintegration. Bigger growing roots exert more pressure on the adjacent rocks. Some plant roots also emit organic acids that help in dissolving the rock’s minerals.

Fig: Biological activity of plant roots causing weathering of rocks Source:https://www.geocaching.com/geocache/GC1JMFF_biological-weathering?guid=a5ac0096-1c88-4307-8fd5-af71868a0c20

Microbial Activity: Some plant microbial activity discharges organic acidic compounds which can break down iron and aluminum minerals into rocks. Microscopic organisms like algae, moss, lichens and bacteria are such kind of plants. They grow on the surface of the rocks and release organic chemicals which have the ability of breaking down the outer layer of the rock by changing the chemical composition of rocks. They release some acidifying molecules (organic acids and protons) and chelating compounds (siderophores and organic acids). The amount of biological activity which results in breakdown of minerals depends on how much life is there in that area. More and more production of these compounds through biological process, results into bio-chemical reactions which  enhances the chemical and physical weathering. The process is also summarized as biological weathering since it is biological in nature. Besides, these microscopic organisms also bring about moist chemical micro-environments which accelerates the chemical and physical breakdown of the rock surfaces.

Burrowing Animals: Burrowing animals such as moles, squirrels and rabbits increases the development of fissures. Many such animals like Piddock shells drill into rocks for defense either by secreting acids to dissolve the rocks or fragment away from the rock grains. These animal activities can make fissures in the rocks and also eats away the rock’s minerals.

As this procedure continues, gaps and holes occur within the rock, thereby exposing the rocks to various chemical, biological and physical weathering. The burrowing animals can move the broken pieces of rock to the surface and thus indirectly enhances the procedure of rock weathering.

Some of the major significant processes involved in biological weathering are:

4. Simply breaking of particles, by utilization of soils particles by animals. Particles may also fracture as a result of animal burrowing as well as by the pressure exerted by growing roots.
5. Movement and mixing of materials- Many huge soil organisms results into the movement of soil particles which can bring together the materials to different weathering processes discovered at discrete locations in the soil profile.
6. Simple chemical procedures like solution can be increased by the carbon dioxide formed during respiration which on mixing with water forms carbonic acid.
7. Chelation is a biological process in which organisms manufacture organic substances, known as chelates, which have the ability of disintegration of minerals and rocks by the removal of metallic cations.
8. Organisms can manipulate the moisture regime in soils and thus increasing weathering. Shade from aerial leaves and stems, presence of roots masses, humus all act to enhance the accessesibility of water in the soil profile. Water is an important component in various physical and chemical weathering processes.
9. Organisms can affect the pH of the soil solution. Respiration from plant roots releases carbon dioxide. Carbon dioxide on mixing with water forms carbonic acid which decreases the soil pH. Cation exchange reactions by which plants absorb nutrients from the soil also results into pH change. The absorption processes generally involves the exchange of basic cations for hydrogen ions. Higher concentration of hydrogen ions results in high acidic soil.

Human Activities: Weathering occurs naturally and slowly but due to human activities it gets accelerated e.g. due to pollution load in the environment by burning coal, natural gas, and oil releases chemicals such as nitrogen oxide and sulfur dioxide into the atmosphere. These chemicals on combining with sunlight and moisture, gets converted into acids. Again come back to earth in the form of acid rain.

Acid rain rapidly weathers limestone, marble, and other kinds of stone. The effects of acid rain can be seen on gravestones. Acid rain has also damaged many historic buildings and monuments .Others human activities such as dig, crash, widen the cracks in rocks by cutting them for constructing buildings dams, roads and canals. The rock’s surface area now gets is exposed may be attacked by chemical agents. This in turn accelerates or speeds up the decomposition of rocks and further wind phenomenon results into fragmentation of rocks. Other activities include mining done by man for different kinds of minerals and metals to extract from the surface of the

Fig-24.7 Human mining activities causing weathering

Source: http://www.srk.com/en/newsletter/rock-engineering-and-slope-stability/

Some other factors in weathering:

There are different types of rocks and hence weathering rates also varies. For ex Igneous rocks tend to weather slowly because they are harder whereas limestone and marble are easily weathered as they can be dissolve easily. Another factor climate i.e temperature and rainfall of a region also influences weathering. As the amount of precipitation increases, so does the rate of solution and the number of chemical reactions. In general, as the amount of rainfall increases, the rate of chemical reaction the asearth from many years. For this hills and the mountains are often being mined by blowing them off. The rock surfaces causes  loosening and disintegration during the blast and further water and air also acts upon these rocks and stones leading to weathering in the longer run. Humans also affect the rates of weathering through removal of plants and vegetation during road construction and house building activities.

Removal of vegetation means exposing soils to high rainfall and consequently run off leaves areas open to chemical and wind weathering. As a result soils lost all their nutrients from the top layers due to excessive amounts of runoff . As the soil being eroded rock which were covered by vegetation become exposed .And now the rocks face all type of pressure of physical weathering, chemical weathering and biological weathering.

well as degree of weathering also increases. On the other hand a dry climate will show the lowest rate of weathering, followed by a very cold climate, irrespective of rainfall it receives. The of highest rate of weathering will be in a wet climate that is also warm or hot. With the rise of the temperature there will be high rate of chemical reactions. And with each 10°C increase in average temperature, double will be the rate of chemical reactions. The region with warmer climate and more types of vegetation the greater the rate of biological weathering and this is due to fact that plants and bacteria grow and multiply faster in warmer temperatures. Hence region’s climate strongly influences the process weathering

Summary:

This module covers the soil forming factors affecting the soil development. Soil development results from the intermesh of 5 factors- climate, Parent material, relief organisms, and time. Weathering is the initial step for many biogeochemical and geomorphic processes. The products of weathering are the leading source of sediments for erosion as well as deposition. Physical weathering causes rocks and minerals to disintegrate. Chemical weathering includes the changes in chemical as well as mineralogical configuration of weathered material. Biological weathering results into disintegration of rock and mineral due to the chemical or physical agents of an organism. The types of organisms that can cause weathering range from bacteria to plants to animals. Weathering is very important for soil formation.

Some important facts

• Mechanical weathering breaks down rocks into smaller pieces without changing their composition
• Mechanical weathering has two important process i.e. Ice wedging and abrasion.
• Chemical weathering breaks down rocks into new minerals having new properties.
• Chemical weathering has three important agents oxygen, water and carbon dioxide
• Different types of rocks have different rates of weathering. More the rocks resistant more it will remain longer.